During the gameplay of the tenth chapter two roles will happen. If I have to chose which of the brothers has longer and more complex task I would point out Thomas.
During covering the second path only for him, he will use the lasso in order to reach higher ledges with nicer views.
The first role will happen when the first wagon (which disappeared when you was killing snipers) will be under attack and William with the woman will run away into the forest.
The woman will run away into the higher part of the forest and Thomas' task will be to find her. Follow her tracks and go up the hill.
You will come across some Indians. Kill them (make use of a bow because they will drop a lot of arrows). On the top of the hill, on the left side the woman will be waiting for you. Move closer to her and trigger a cut-scene.
When the conversation will go to the end follow another star (no one will attack you in that moment of the gameplay). You will reach the rest of McCall family.
The second unique task for Thomas will start at the end of the chapter after leaving a big and beautiful cove. This time Thomas will be asked to follow among the left path, Ray will go into the right one.
Make use of the lasso and cover the precipice and continuing your way go through the fallen three. Right after it Indians will attack you.
There won't be a lot of them and it will be easy to make out them so you shouldn't have any problem with eliminating them. On the top of the hill (another great view) look down towards the campfire. The last enemies will wait them for death. Kill them with the cooperation with Ray.
Then use the lasso (and the dead three from the picture) and join him at the bottom of the cliff.
Ray's tasks are nearly as complex and long as Thomas'. Two times you will separate from your brother.
But playing as Thomas, during the second role you will get to the better views and that is why I recommend the weaker brother.
During the first task for Ray, in a place where the first wagon will be attacked (after disappearing) your companions will run away to the nearest forest. Playing as Ray follow William down the hill (left side).
The way to cover won't be long, the spiritual McCall will be behind the nearest rocks and on your way there a few savages will attack you (they won't be hidden).
After joining with your brother and the short conversation go back with him to the place where they split up. Then go up the hill towards Thomas.
In that way the three brothers and the woman will find themselves together once again.
The second split will happen at the end of the chapter after leaving a big and beautiful cave. This time Ray will take the right path and will go up the hill.
You will reach the place where Indians will be waiting for you. Eliminate them (you can do this before the second McCall brother will help you). There won't be a lot of the savages. Right after the last death of the enemy brother will join together.